gnu: artanis: Remove implied $(DESTDIR) usage.

* gnu/packages/patches/artanis-fix-Makefile.in.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register patch.
* gnu/packages/guile.scm (artanis)[source]: Use patch.
[arguments]: Remove DESTDIR from #:make-flags.

Signed-off-by: Marius Bakke <mbakke@fastmail.com>